These devices comprise a pulse generator that is able to supervise (monitor) the cardiac activity of a patient and to generate shock pulses of high energy, which are applied to the heart when the heart presents a ventricular arrhythmia that is susceptible to be treated by a shock therapy. The pulse generator also is typically able to generate a stimulation pulse for stimulating a heart beat in the absence of a spontaneous contraction in one or both of the atrium and the ventricle. When the shock pulse of high energy is between approximately 0.1 and 10 J, one designates this shock therapy by the name of "cardioversion", and the electrical shock is called a "cardioversion shock." When the energy is greater than approximately 10 J, one designates this shock therapy "defibrillation" and the electrical shock is called a "defibrillation shock".
The high energy shock is to be delivered when one detects a ventricular tachycardia (VT), but only when it concerns a real VT, and not a supra-ventricular tachycardia (SVT). Indeed, in the case of an SVT, the tachycardia is of atrial origin and the shock that would be delivered to the ventricle would be without effect because the defibrillation electrode, or if needed the stimulation electrode, are not placed in the region of the atrium.
A tachyarrhythmia (also called a tachycardia) corresponds to an abnormal rapid cardiac rhythm and covers ventricular fibrillation (VF), ventricular tachycardia (VT), sinus tachycardia (ST) and supra-ventricular tachycardia (SVT). The supra-ventricular tachycardia (SVT) includes atrial tachycardia, atrial flutter, and atrial fibrillation. The diagnosis of a tachycardia is operated, in a manner that is known (see particularly, for example, EP-A-0 626 182 in the name of ELA Medical and its corresponding U.S. Pat. No. 5,462,060, which is incorporated herein by reference), from criteria such as the ventricular frequency, the ventricular stability interval (RR interval), the analysis of the atrial-ventricular association, and the mode of starting of the tachycardia (that is, the presence of acceleration and the chamber of origin (ventricular or atrial)).
The diagnosis algorithm that provides for the detection of tachycardias and their classification (that is to say a discrimination between SVT and VT) is conceived in a manner to obtain a maximal sensitivity - to detect all ventricular tachycardias, thus avoiding the false negative diagnosis - while preserving the specificity of the discrimination - that is to say by accurately discriminating between SVT and VT, thus avoiding the false positive diagnosis. A false positive is an indication of a VT when it actually is an SVT. Further, this diagnosis algorithm allows to detect atrial fibrillations (AF), that is to say abnormally high frequencies of the atrial rhythm, and allows to distinguish an isolated AF (one that should not result in the delivery of a ventricular shock therapy) from a VT (one that should result in a rapid delivery of a ventricular shock therapy).
One observes, however, in some cases, a failure of the classification algorithm, particularly in the presence of an installed and conducted AF presenting regular RR intervals during a sufficiently long duration, which result in a false diagnosis of VT (i.e., a regular rhythm which is dissociated between the atria and ventricle). If one decreases the sensitivity of the classification algorithm to avoid such false diagnosis, one risks, on the one hand, to end the diagnosis algorithm without a detection of VT, and on the other hand, a lengthening of the analysis duration of the diagnosis algorithm.
The disadvantages are, in the first case, the risk of not detecting a real VT, and, in the second case, a lengthening of the period between the detection of the VT and the application of the appropriate shock therapy. The lengthening of the period is adverse to the well-being of the patient.